"We were blown away," said biologist Mark Norman of discovering the octopus behavior off Indonesia. "It was hard not to laugh underwater and flood your [scuba] mask."

The coconut-carrying behavior makes the veined octopus the newest member of the elite club of tool-using animals—and the first member without a backbone, researchers say.

Coconuts to Go

A team led by biologist Julian Finn of Museum Victoria in Melbourne, Australia, was observing 20 veined octopuses (Amphioctopus marginatus) on a regular basis.

The researchers noticed that the animals were frequently using their approximately 6-inch-long (15-centimeter-long) tentacles to carry coconut shells bigger than their roughly 3-inch-wide (8-centimeter-wide) bodies.

An octopus would dig up the two halves of a coconut shell, then use them as protective shielding when stopping in exposed areas or when resting in sediment.

This, on its own, astonished the team. Then they noticed that the octopuses, after using the coconut shells, would arrange them neatly below the centers of their bodies and "walk" around with the shells—awkwardly.

"I've always been impressed by what octopuses can do, but this was bizarre," said study co-author Norman, senior curator for mollusks at Museum Victoria.

To carry the shells, a veined octopus has to stick its arms out and over the edges of the coconut and walk around as if on stilts—making the octopus, while in motion, more vulnerable to predators—study leader Finn explained.

"An octopus without shells can swim away much faster by jet propulsion," he said. "But on endless mud seafloor, where are you fleeing to?" In other words, a coconut-carrying octopus may be slow, but it's always got somewhere to hide.

So what makes the veined octopus's behavior tool use, versus, say, the hermit crab's use of seashells as armor?

Worn nearly constantly, a hermit crab's adopted shell isn't considered a tool, because it's always useful. Tools, by definition, provide no benefit until they're used for a very specific purpose—showing that the animal is capable of what you might call advance planning.

Octopuses of many species are well known for their intelligence. In captivity they've been known to navigate mazes, seem to be able to remember past events, and are cunning escape artists. (See "Curious Octopus Floods Aquarium.")

Related Video: Octopus Navigates Skinny Maze

Octopus Tool Use Especially Surprising

Tool use, once thought to be a uniquely human behavior, is seen as a sign of considerable mental sophistication among nonhuman animals.

"I really wasn't expecting to see tool use appear in cephalopods"—squid, cuttlefish, and octopuses—said biological anthropologist Craig Stanford, co-director of the Jane Goodall Research Center in Los Angeles, who wasn't involved in the new study.

That the octopuses weren't using their tools to rustle up dinner only added to Stanford's surprise. "Even chimps," he added, "do not use natural materials to create shelters over their heads."